In X-ray image acquisition, X-rays emitted from an X-ray source are usually directed to an object to be examined and X-rays transmitted through and partly absorbed within the object are then detected on a detection surface of an X-ray detector.
Conventionally, an X-ray image acquisition device comprises a collimator. The collimator may comprise non-transparent closure parts for completely blocking portions of an X-ray beam coming from the X-ray source. Furthermore, a semitransparent X-ray transmitting device for example in the form of semitransparent diaphragm wedges may be provided. The non-transparent closure parts and the semitransparent X-ray transmitting device may enable to shape the X-ray beam coming from the X-ray source in such a way that only parts of a patient's body that are of interest are irradiated with a desired radiation intensity. Furthermore, the collimator may comprise filter elements in order to vary the spectrum of the beam in a desired way.
Using a collimator may lead to several advantages such as improving an image quality, reducing a risk of radiation damage to a patient and reducing of scattered radiation to which for example staff is exposed during image recording. In this context, the reduction in radiation exposure might be important particular in view of prolonged diagnostic and therapeutic interventions accompanied by X-ray fluoroscope observations that are increasingly taking place.
Conventionally, the collimator and in particular the semitransparent wedges have been predominantly adjusted manually by the attendant staff. However, this is not only cumbersome but may also distract from e.g. the actual surgery activity.
In order to simplify the utilization of adjustment possibilities of a collimator of an X-ray image acquisition device, U.S. Pat. No. 7,340,033, which is assigned to the same applicant as the present application, discloses an X-ray unit for generating imaging of a body comprising an automatically adjustable collimator including diaphragm and filter elements for limiting, locally attenuating and/or filtering an X-ray beam output from an X-ray source. Therein, the X-ray unit further comprises a data processing unit coupled to the collimator and designed to localize a region of interest inside a body and to transmit commands to the collimator to adjust the diaphragm and filter elements of the collimator in accordance with a restriction of subsequent X-ray beams to a localized region of interest.
The automatic adjustment of the collimator and, in particular, the automatic positioning of a semitransparent wedge usually requires a determination of an optimum position of the wedges based on some detected region of interest which shall be examined and to which the X-ray beam may be collimated. In order to be able to automatically detect a region of interest it might be necessary to use an automatic feature detection algorithm. A preliminary X-ray image may be acquired for example with a non-restricted X-ray beam and the automatic feature detection algorithm may detect a region of interest within the acquired image. Knowing the location of the region of interest, an optimum wedge position might be calculated and sent to the collimator. Thus, the wedge may be positioned such that portions of the X-rays from the X-ray source which would be transmitted through the examined object outside the region of interest are at least partly attenuated. Thereby, for example the overall X-ray dose to a patient might be reduced.
However, after wedge positioning, the automatic feature detection may not be able to detect the features of the region of interest any more. It may happen that the object to be examined changes its position with respect to the X-ray image acquisition device after such initial wedge positioning. For example, a patient to be examined might move or a table, on which the patient lies, might move with respect to the X-ray image acquisition device. Accordingly, even though the collimator and the wedges have been initially correctly positioned, such positioning might not be convenient any more during the actual observation, for example during a prolonged diagnostic or therapeutic intervention. Accordingly, the X-ray beam might not be collimated to the region of interest at all times thereby for example preventing correct image acquisition or deteriorating image quality.